Continuous preparation of polyamides from aminonitriles

Abstract

A continuous process for preparing a polyamide by reacting at least one aminonitrile with water comprises:(1) reacting at least one aminonitrile with water at a temperature from 90 to 400° C. and a pressure from 0.1 to 35x106 Pa in a flow tube containing a Brönsted acid catalyst selected from a beta-zeolite catalyst, a sheet-silicate catalyst or a titanium dioxide catalyst comprising from 70 to 100% by weight of anatase and from 0 to 30% by weight of rutile and in which up to 40% by weight of the titanium dioxide may be replaced by tungsten oxide to obtain a reaction mixture,(2) further reacting the reaction mixture at a temperature from 150 to 400° C. and a pressure which is lower than the pressure in step 1, in the presence of a Brönsted acid catalyst as in (1), the temperature and pressure being selected so as to obtain a first gas phase and a first liquid or a first solid phase or a mixture of first solid and first liquid phase, and the first gas phase is separated from the first liquid or the first solid phase or from the mixture of first liquid and first solid phase, and(3) admixing the first liquid or the first solid phase or the mixture of first liquid and first solid phase with a gaseous or liquid phase comprising water at a temperature from 150 to 370° C. and a pressure from 0.1 to 30x106 Pa to obtain a product mixture.

Description

The present invention relates to a continuous process for preparing polyamides from aminonitriles and water at elevated temperature and elevated pressure.PRIOR ARTU.S. Pat. No. 4,629,776 describes a catalytic process for producing polyamides from .omega.-aminonitriles such as .omega.-amino-capronitrile (ACN). ACN is reacted with water in the presence of a catalytic amount of an oxidized sulfur compound as catalyst. Sulfuric acid is an example of the catalyst used.U.S. Pat. No. 4,568,736 describes a similar catalytic process for producing polyamides. The catalyst used in this case is an oxygen-containing phosphorus compound, phosphoric acid or a phosphonic acid.Complete removal of catalyst is virtually impossible in either process. The presence of catalyst in the polymer can hinder the building of high molecular weight polymers and compromise later processing operations, for example spinning. Moreover, the level of volatiles in the polymers obtained is high, so that the polyamides ar...

AI Technical Summary

Benefits of technology

Preferably, the pressure is adjusted at a preselected temperature so that the pressure is smaller than the equilibrium vapor pressure of ammonia, but greater than the equilibrium vapor pressure of the other components in the synthesis mixture at the given temperature. This way, it is possible to favor especially the removal of ammonia and thus speed up the hydrolysis of the acid amide groups.

The pellet form of the catalyst makes it possible to remove the catalyst mechanically at the point of exit from the first step. For example, mechanical filters or sieves are provided at the point of exit from the first step, for example. If the catalyst is additionally used in the second and / or third step, it is preferably present in the same form.

Preferably, the reactor of the second step likewise contains the catalyst material of the invention, especially in pellet form. The reactor provides a further improvement in product properties compared with a catalyst-free reactor especially at higher pressures and / or in the presence of a large amount of excess water in the reaction mixture. Temperature and pressure should be selected in such a way that the viscosity of the reaction mixture remains sufficiently small to prevent any blinding of the catalyst surface. According to the invention, the point of exit from the second process step too is equipped with sieves or filters which guarantee the purity of the reaction mixture and separate the catalyst from the reaction mixture.

In step 3, the first liquid or the first solid phase or the mixture of first liquid and first solid phase is admixed with a gaseous or liquid phase comprising water, preferably water or water vapor. This is done continuously. The amount of water added (as liquid) is preferably within the range from about 50 to about 1500 ml, more preferably within the range from about 100 to about 500 ml, based in each case on 1 kg of the first liquid or first solid phase or of the mixture of first liquid and first solid phase. This addition of water primarily compensates the water losses incurred in step 2 and furthers the hydrolysis of acid amide groups in the synthesis mixture. This results in a further advantage of this invention, that the mixture of the starting materials as used in step 1 can be used with a small excess of water only.

The pressure can be adjusted at a preselected temperature so that the pressure is smaller than the equilibrium vapor pressure of ammonia, but greater than the equilibrium vapor pressure of the other components in the synthesis mixture at the given temperature. This way, it is possible to favor especially the removal of ammonia and thus speed up the hydrolysis of the acid amide groups.

The fourth step too can be operated using the catalyst of the invention. The use of the catalyst in step 4 improves the molecular weight buildup especially when the relative viscosity of the effluent from the third or--in the case of the three-step procedure--second step is less than RV=1.6--and / or the molar nitrile group and acid amide content in the polymer is greater than 1%, each based on the moles of aminonitrile used.

Problems solved by technology

Complete removal of catalyst is virtually impossible in either process.

The presence of catalyst in the polymer can hinder the building of high molecular weight polymers and compromise later processing operations, for example spinning.

Moreover, the level of volatiles in the polymers obtained is high, so that the polyamides are difficult to process.

Existing processes have in some cases inadequate space-time yields and a molecular weight buildup which needs to be improved.

In addition, the product is not always obtained in the requisite purity.

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